1. Field of the Invention
This invention relates to a pneumatic radial tire having low rolling resistance and a method of manufacturing the same. More particularly this invention relates to a tire adapted to improve various tire performances, inter alia to effectively reduce the rolling resistance by properly modifying a radial surface profile of a carcass.
2. Description of the Prior Art
It is known that in order to reduce the rolling resistance of a tire it is necessary to reduce the energy which is to be consumed due to periodic stresses and strains caused in the tire while rolling. It is a reasonable assumption that rates of consumed energies at respective portions of tires are approximately 34% at treads, 27% at buttress portions, 25% at sidewalls and 14% at bead portions according to a result of analysis on percentages of consumed energies of so-called radial carcass tires whose practical usefulness has been recognized under normal used conditions that is, inflated at internal pressure of the order of at the most 2 kg/cm.sup.2.
The tread has the largest contribution to the rolling resistance of the tire, and in order to reduce inner friction and hence the rolling resistance, therefore, the compounding ratio of tread rubber has been generally selected so as to increase its resilience. In this case, however, wet performance, which is one of the important performances of the tire, is adversely affected undesirably depending upon the degree of the reduced rolling resistance.
With the above solution, accordingly, the rolling resistance cannot be greatly decreased unless a particular precaution is taken to prevent the wet performance from being lowered. Since an effective method to maintain the wet performance has not been found yet, the above solution does not achieve a significant effect.
It has been further proposed to use for sidewalls the rubber compounding ratio having reduced inner friction in the same manner as in the tread. This proposal, however, serves only to reduce 3% or less of the rolling resistance but adversely affects its damping capacity for vibration caused in the tire and hence its important riding comfortability.
Moreover, it has been proposed to change two layer carcasses into single layer carcasses or to narrow belt widths to make light weight tires in order to reduce the rolling resistance of the tire. However, such proposals unavoidably reduce the rigidity of important portions of the tires to decrease the steering stability, so that only limited effects can be expected.